1. Field of the Invention
The present invention relates generally to casting and more particularly to encapsulation of items within thermosetting polymeric materials.
2. Description of the Prior Art
Cast encapsulation of items, particularly electronic components, is a well established packaging technology. Currently, such items are encapsulated by a process called transfer molding. In this process, a thermosetting encapsulation material is retained within a chamber in a transfer molding machine while an item to be encapsulated is secured within a molding cavity in another portion of the machine. The chamber containing the thermosetting encapsulant and the molding cavity are connected by one or more passages so that pressure applied to the thermosetting encapsulant may cause it to flow into the molding cavity and around the item to be encapsulated. After the thermosetting encapsulant has solidified, the item, now encapsulated, may be removed from the molding cavity.
The encapsulation material often used in the transfer molding process is an epoxy resin mixed with a particulate filler such as silica or alumina. These particulate materials are incorporated into the encapsulation material principally to increase its thermal conductivity. The greater the concentration of particulate filler in the encapsulation material, the higher its thermal conductivity. However, addition of this particulate material to the thermosetting resin also increases its viscosity, thereby creating a corresponding increase in the force required for transfer molding.
There are several problems with the transfer molding process as currently employed to encapsulate items such as electronic components. First, because high pressures are required to transfer the thermosetting encapsulant into the molding cavity, that cavity must be fabricated from metal. Since the thermosetting encapsulant materials such as epoxy resins normally bond well to metal, the general practice in the industry is to incorporate a mold release compound into the encapsulant material so that after it has solidified the encapsulated item may be easily removed from the molding cavity. However, since the electrically conductive leads by which electrical currents flow into or out of an electronic component are generally made of metal, these mold release compounds may also permit the encapsulation to separate from the electrically conductive leads. Since contact with air, particularly the moisture in air, often degrades the performance of electronic components, the detachment of the encapsulation from the electrically conductive leads provides a path by which moisture may reach an encapsulated component and thus may contribute to its failure.
A second problem with transfer molding as currently practiced in encapsulating electronic components is referred to as "wire sweep." The term "wire sweep" describes the bending of electrically conductive leads by the force of inflowing encapsulation material. Since the electrically conductive leads of electronic components being encapsulated are generally quite fragile and must remain electrically insulated from each other after encapsulation, "wire sweep" establishes an upper limit on the rate at which encapsulation material may be transferred into the molding cavity. Furthermore, this rate of transfer decreases as the viscosity of the encapsulation material increases such as occurs with increasing concentrations of particulate filler. Therefore, the current transfer molding process involves a trade-off between the thermal conductivity of the encapsulation material and the speed with which the molding cavity may be filled without damaging the item being encapsulated.